1. Field of the Invention
This invention relates to signal processing circuits.
2. Description of the Prior Art
An "infinite Q circuit" is a description that is normally associated with a circuit characterized by a pole in the frequency domain that is located exactly on the j.omega. axis. As a consequence of this pole location, an infinite Q circuit responds to an input signal of a unit impulse by developing sinusoidal oscillations of constant amplitude at the frequency of the circuit's pole. If a source of energy is available in the circuit for replenishing withdrawn power, the circuit can serve as a single-frequency oscillator.
As a further consequence of the pole location, an infinite Q circuit responds to a continuous input signal by developing an output signal that is continually increasing with time when the applied input signal is of a frequency equal to the frequency of the circuit's pole. The infinite Q circuit, thus, is a tuned filter, and can serve as a signal detector.
In practice, sine wave analog oscillators are generally designed to possess a frequency domain pole that is slightly to the right of the j.omega. axis to assure continued oscillation. Unfortunately, a pole located to the right of the j.omega. axis causes the oscillator's output signal to increase in magnitude until circuit nonlinearities prevent a further magnitude increase. These nonlinearities distort the output signal and thereby introduce unwanted harmonics.
Digital oscillators generally use binary arithmetic in combination with various forward acting and backward acting feedback paths. Because of the finite capacity of conventional arithmetic digital circuits, however, truncation of signals must occur. The truncation causes distortions in the output signal, with results similar to those of analog oscillators.
Similar practical problems exist in the filter art. Analog filters suffer from the instabilities and inaccuracies characteristic of all analog circuits, and digital filters require relatively large storage capacity in order to assure proper sensitivity and selectivity of the filter and proper safeguard against computation errors which cause undesirable limit cycles.